Definition
Published minimum altitudes on instrument procedures and charts that guarantee a specified vertical buffer above terrain and man-made obstacles within a defined area. Examples include the Minimum Obstruction Clearance Altitude (MOCA), Minimum En Route Altitude (MEA), Minimum Vectoring Altitude (MVA), Off-Route Obstruction Clearance Altitude (OROCA), and the Obstacle Clearance Altitude/Height (OCA/H) on approach charts. Each type guarantees a different buffer over a different area, but all serve the same purpose: keeping the aircraft safely above obstacles when the pilot cannot see them.
Plain English
These are the lowest altitudes you are allowed to fly on a given route or procedure that still keep you safely above the ground and any towers, mountains, or buildings underneath you. Different charts and procedures use different versions of this altitude, but all of them are designed so that if you stay at or above the published number, you will not hit anything.
Context Anchor
Seen during IFR preflight planning when checking route altitudes, approach information, and terrain or obstacle risks along the planned flight path.
Derivation
Obstacle comes from a Latin idea meaning something that stands in the way. Clearance means space kept free between things. Altitude comes from Latin altus, meaning high. Together, the phrase points to a height that keeps the airplane clear of things standing in its path.
Why Pilots Care
Maintains the required safety buffer from obstacles when visibility is low or the terrain is hidden.
Grounding Statement
If you cannot see the ground ahead, obstacle clearance altitudes help you plan a height that keeps the airplane above what is there.
Intuition Check
Clearance here does not mean permission from ATC. It means physical space between the aircraft and obstacles.
Example Sentence 1
During preflight, she checked the obstacle clearance altitudes along her route and noted the highest MEA was 11,000 feet through the mountain segment.
Example Sentence 2
We climbed to the published obstacle clearance altitude before turning toward the first fix.